Disposable absorbent articles, in particular, disposable diapers are well known articles of manufacture which are designed to be worn principally by infants and incontinence sufferers. Such diapers are worn about the lower torso of the wearer and are intended to absorb and contain urine and other bodily discharges, thus preventing the soiling, wetting, or similar contamination of articles (e.g., clothing, bedding, other persons, etc.) that may come into contact with such a diaper in use. In recent years, disposable diapers in the form of pull-on diapers have emerged on the market and in particular, disposable pull-on diapers with fixed sides have gained in popularity. Typically, the fixed sides of the disposable pull-on diaper are manufactured by joining the side panels of the front portion to the side panels of the rear portion. For joining purposes, the contacting surfaces of the side panels need to be at least partially melted. It is, however, desirable to avoid melting the outer surfaces of the side panels corresponding to the area to be joined. In general, a technique which permits the joining of several layers of thick material that does not lead to the formation of hard, raspy protuberances on the outer surfaces is required. It has been documented that current techniques are more easily suited to the joining of thin layers of material. Therefore, the problem of joining thick materials is known in the art but the only solutions proposed hitherto, as far as the present applicants are aware, are those described hereforth.
Typical of prior art processes are the conventional thermal joining processes, which utilise hot pins made of steel, aluminium and copper or other materials with a high coefficient of thermal conductivity to transfer the required energy for melting into the webs to be joined. For thin materials like films, the hot pins touch and melt the outer surfaces of the web structure. For thicker materials, it is preferred that the hot pins penetrate the whole web structure. The upward and downward action of the hot pins results in both the creation of holes with molten walls and some of the melted material being displaced and transported to the surface of the web structure to form--after cooling--hard, raspy protuberances. In order to ensure that an effective joining takes place, compression of the web structure subsequently follows. Nevertheless, before the compression tools can be placed in position, the hot pins have to be moved away resulting in a complex operation. It is only when the melted material has cooled that the compression tools are removed from the web structure.
The perforation of a film material generally, but not exclusively of, thermoplastic material is taught in U.S. Pat. No. 4,667,552. Heated perforation pins are carried by a rotating cylinder and the film to be perforated is forced against the pins by a pressure means such as a deformable roller. The hot pin enters from the top of the plastic film and as the hot pin melts the surrounding film after the pin perforates the film, the film is melted into an `upset` structure. The film, after perforation, has one smooth face and an obverse face that has a texture due to the proliferation of protuberances. The technique is not suitable for the joining of thick materials.
U.S. Pat. No. 4,519,798 typifies a process that utilises thermal energy to soften two or more layers of thermoplastic material for the purposes of joining. The patent discloses a disposable diaper structure wherein an absorbent core is encapsulated between a multiplicity of sheets. Each of the sheets is constructed of a material, such as polyethylene, which will heat seal without the necessity of an adhesive. The diaper is made by heating and sealing the overlapping edges of the polyethylene sheets directly to one another outside the borders of the absorbent core.
The autogenous lamination of plural layers or laminae of sheet material are described in U.S. Pat. No. 4,919,738. In more detail, the patent teaches a method of dynamically bonding plural laminae together, at least one of which laminae comprises thermoplastic material. The lamination is achieved through the use of pressure biased laminating rolls which are operated with a predetermined surface velocity differential between them. Indeed, in some lamina, portions of the bonding sites may protrude resulting in hard, raspy protuberances rather than being recessed.
Layers of thermoplastic materials are welded together by means of ultrasonic vibration equipment in U.S. Pat. No. 3,733,238. A plurality of spaced ultrasonic vibration transmitting members having working surfaces in direct contact with one side of the sheet-like elements is employed in co-operation with opposing spaced anvil surfaces located on the opposite side of the sheet-like elements to produce full width webs of thermoplastic laminated material. As is evident, the technique requires that the layers of thermoplastic material are always in direct physical contact with the mechanical energy transfer tools, thus leading to contamination of the webs, machine wear and thus machine inefficiency.
A seam composed of a six-layered structure comprising the outer fabric layer, the inner impermeable layer of the cover sheet and the permeable layer forming the liner of the garment is disclosed in U.S. Pat. No. 4,610,681. In order to form a small and unobtrusive bond, the ultrasonic sealing is accomplished with a plurality of lines that form pressured areas and raised line areas. As above, the technique is mechanical in nature and results in contamination of the webs, machine wear and machine inefficiency.
A means of joining thick layers of material is outlined in U.S. Pat. No. 4,909,804, which discloses seams of a disposable training pant that have been joined by stitching.
WO 96/19313 describes a method for forming through apertures in the form of holes and/or slits in a web that is intended to form part of an absorbent article, e.g., the topsheet of an absorbent article being apertured to obtain liquid permeability. According to the invention, a web is irradiated with at least one focused electromagnetic beam or particle beam from an irradiating source on at least one of its surfaces and in those web regions where the apertures are to be formed. During irradiation the web can be in contact with another web which includes material of a kind similar to the first web and the properties of the beam and the duration of the irradiation period are chosen so that the material in the first web and/or in the further web will be supplied with sufficient energy to join the webs in the immediate vicinity of the apertures. A fluid may be delivered to the vicinity of the focusing point on the web to remove molten/burned/vaporised material from holes made in the web. In spite of the lack of contact with the webs to be joined, the technique focuses on aperturing and results in melted material being displaced and transferred to the topsheet surface.
Needlepunching is a mechanical bonding technique which is utilised to join webs of material. In U.S. Pat. No. 5,397,632, non-woven webs for use in an improved automotive depth air filter are mechanically secured at intermittent locations. During the needlepunching operation, a plurality of needles having a fibre advancing configuration are passed through all the webs. The various webs thus become mechanically interconnected and interlocked through the entanglement of staple fibres and filaments. During such needle insertion and withdrawal, staple fibres from the upper web and relatively mobile fibres from the intermediate web are driven forward and into the lower web. The surface of the final product is not smooth to the touch.
As a result of the above prior art attempts, it has been recognised by those skilled in the art that it would be desirable to provide a method of joining thick webs for particular use in disposable absorbent articles that eliminates the presence of hard, raspy protuberances on the outer surfaces caused by the solidification of melted web material; that does not require the use of heat transfer tools that come into physical contact with the webs to be joined; that at least partially melts the meltable components in the web structure to render a sufficient tackiness for joining; and that results in a product with superior tactile properties and concomitant strength, resistance to wear and breathability characteristics. The solution was found to be a method wherein a high speed jet of heated fluid is directed into the outer surface of at least two thick and porous webs at discrete locations and wherein the constituent meltable components are at least partially melted in the area of overlap of the web structure.
It has now been discovered that the benefits of the present invention range from a method that enables the joining of thick, porous webs in an extremely effective manner; to a method that eliminates the irritating hard and raspy protuberances of the prior art techniques; to a method that does not rely on heat transfer tools coming in contact with and contaminating the web structure as is typical of conventional joining processes; to a method that minimises process time; to a method that leads to improved machine efficiency due to reduced wear and friction; and to a product with enhanced tactile qualities and superior resistance to wear characteristics leading to improved consumer satisfaction and confidence.